This invention relates generally to semiconductor devices and, more particularly, to semiconductor devices used to provide a precise voltage for use as a stable, precise voltage reference.
As it is known in the art, precision voltage references (PVRs) are used in electronic systems which require a stable reference voltage. Circuits such as analog-to-digital and analog-to-frequency converters, calibration standards, and precise current and voltage sources generally require the use of precision voltage references. Moreover, in many military applications, precision voltage references must provide a voltage which remains within stringent limits after exposure to a specified radiation environment in order for the system employing such a device to fulfill its mission. In most instances with exposure to a radiation environment, the so-called fast neutron component of the environment is the primary concern. Other concerns, however, are total dose and dose rate effects of gamma and x-ray exposures on the precision voltage reference. Generally, exposure to such environments either can destroy a conventional precision voltage reference or more likely cause short-term, as well as permanent long-term variations in the reference voltage provided from such a device after exposure to the radiation environment. A second requirement is that often times the precision voltage reference must operate over varying temperature ranges and accordingly the device must be temperature compensated to maintain a substantially constant voltage over the operating temperature range of the device.
A conventional, precision voltage reference includes a reverse biased silicon avalanche diode which is typically soldered to a forward biased silicon junction. The temperature coefficients of the two diodes are large in magnitude but opposite in sign. By careful design and component selection, it is generally feasible to provide a composite device with a temperature coefficient smaller than 5 ppm/.degree.C. The stability of the conventional precision voltage reference in a neutron radiation environment relies upon a compensating mechanism in the reverse biased and forward biased diodes. Fast neutrons incident on the silicon devices create clusters of displaced silicon atoms. In the avalanche reverse biased diode, the primary effect of these defect clusters of silicon atoms is to provide intermediate energy levels to remove majority carriers. As a result, a junction depletion layer, which is present in the reverse biased avalanche diode widens, and thus the avalanche voltage increases for a fixed terminal current. Neutron damage to the forward biased junction, however, provides a decrease in minority carrier lifetime which causes a decrease in the forward voltage drop or forward junction voltage across the junction as a function of increasing radiation. Generally, the variation in forward voltage caused by the decrease in minority carrier lifetime is substantially greater than the changes in reverse voltage caused by effective widening of the junction depletion region in the reverse biased diode. The radiation induced decrease in the forward biased junction voltage and hence in the PVR is mitigated in the conventional precision voltage reference by doping the junction with gold during the fabrication process to purposely reduce the minority carrier lifetime. At low levels of gold doping, the voltage stability of the device is limited by the strong decrease in the forward junction voltage with neutron radiation. As gold doping is increased, however, compensation over a broader range of neutron fluences, is provided.
One problem with the above-described device, however, is that neither the control of the gold doping process, nor the matching of the neutron fluence dependencies of the avalanche diode and the pn junction provide sufficient stability adequate for the most demanding modern military systems.
Another problem is that the gold doping process is not suitable for fabricating a monolithic device since it is not practical to confine the doping to a specific junction. Therefore, the conventional approach requires the separate fabrication of the diodes with their eventual hand soldering to provide a practical device. This approach is expensive, has low reliability, as well as low reproducibility.